Anodic films are metal oxide layers that are integrally formed on anodizable metals such as aluminum and aluminum alloys. The anodic films are formed by exposing a metal substrate to anodizing process, whereby a portion of the substrate is converted to its corresponding metal oxide. Anodic films are generally hard and resistant to corrosion, and are therefore widely used in industry to provide durable thin coatings to outer surfaces of parts.
One of the challenges associated with anodic films relates to the differences in thermal expansion of the anodic film, which is generally amorphous similar to glass, and the underlying metal substrate. In particular, the metal substrate will expand more than the anodic film when exposed to heat. As a consequence, when the metal substrate and anodic film cool down, the metal substrate will shrink more than the anodic film, causing micro-cracks to form within the anodic film. This can happen, for example, when a laser marking process locally heats the metal substrate, forming micro-cracks in the anodic film above the laser marked area of the substrate. In some applications, these micro-cracks are not large enough to cause cosmetic or functional problems. However, in other applications, these micro-cracks act as entry points for moisture and other contaminants that can reach the underlying metal substrate and cause corrosion products to form.